Integrated circuits are used in a wide variety of electronic devices and consumer products, from computers and computer peripherals to automobiles, electronic games, toys, machine tools, medical and scientific instruments, and other products and devices. Integrated circuits are essentially extremely complex and highly miniaturized electronic circuits. Modern integrated circuits routinely including millions of transistors that implement complex logic circuits to provide processor and memory devices with complex interfaces and functional behavior.
In general, integrated circuits are manufactured in high volumes by extremely expensive and technically demanding fabrication processes. In many ways, integrated circuits fabrication is one of the first, and by far most commercially successful, applications of nanotechnology. The smallest features of currently available integrated circuits have widths between 100 and 200 nanometers. Integrated circuits are commonly fabricated using photolithographic techniques that focus light through images of circuit patterns onto chemically prepared substrates in order to reduce the microscopic-scale images down to the submicroscopic patterns from which features of integrated circuits are constructed, layer by layer. Each step in the integrated circuit fabrication process involves careful deposition of metallic films, dopants, and photoresist, mechanical planarization, application of reactive etchants, rinsing with solvents, and other operations. Each photolithographic step requires careful alignment of a current photolithography mask with the features constructed in previous photolithography steps. Because of the complexity of the integrated circuit manufacturing process, and the small dimensions of integrated circuit features, extreme measures are taken to prevent contamination of nascent integrated circuits, and the fabrication environments in which they are manufactured, by dust, chemical contaminants, and other environmental interferences. However, despite these measures, and despite the high tolerances and extreme precision of integrated circuit manufacturing tools and procedures, a relatively large proportion of finished integrated circuits are subsequently found, during post-manufacturing testing, to be defective. As a result, relatively large proportions of the integrated circuits produced in an integrated circuit fabrication facility are unusable for their intended applications, and discarded. Each discarded, defective integrated circuit represents the loss of significant amounts of energy and highly purified chemical components, as well as the loss of significant amounts of time and, ultimately, money. For this reason, manufacturers of integrated circuits, as well as integrated circuit vendors and those who purchase and use products containing integrated circuits, have all recognized the need for decreasing the rate of defective integrated circuits, or, in other words, increasing the yield of functional integrated circuits, and, by doing so, decreasing the cost of integrated circuits.